Why isn't mercury in synchronous rotation with respect to the Sun?

Answer 1

The Sun created a significant tidal bulge on Mercury when it was still a young, molten planet; when the planet solidified, this bulge was locked in place. Mercury is so close to the Sun that the Sun's gravitational force on Mercury's tidal bulges changed the planet's rotation rate. However, Mercury's highly eccentric orbit has prevented the planet from getting locked into synchronous rotation.

Answer 2

Because of solar tides.

Improved answer : 1) Solar tidal forces would help to cause synchronous

rotation not explain why it's not synchronous.

2) There is a definite ratio between Mercury's orbital

period and its rotation period of 3:2.

So the rotation is not synchronous, but it is "semi synchronous".

I've just looked up the scientific wording for Mercury's behaviour :

it's called "locked in a 3:2 spin-orbit resonance".

Mercury rotates 3 times for every 2 orbits. Planetary scientists say this is

because the Mercury-Sun distance varies a lot during each orbit. So the

Sun's tidal effect on Mercury varies a lot, preventing normal synchronous

behaviour.

Answer 3

Mercury is not in synchronous rotation with the Sun because its rotation period (59 Earth days) is not the same as its orbital period (88 Earth days). This means that different parts of Mercury's surface experience different amounts of sunlight, leading to extreme temperature variations between its day and night sides.